It is known to produce cobaltic hydroxide-containing precipitates as a by-product in the hydrometallurgical treatment of nickel-bearing materials, such as oxidic nickel ores or nickel sulfide concentrates or mattes. Nickel and cobalt are usually found together in natural-occurring minerals and, because conventional ore dressing methods do not effect a separation of these two elements, both metals generally appear together in solutions resulting from the leaching of nickel and cobalt-containing materials, such as in the leaching of nickeliferous oxide ores or the oxidation leaching of nickel sulfide concentrates or mattes.
In recent years, several hydrometallurgical methods have been proposed for the recovery of nickel and cobalt from lateritic ores or from nickel and nickel-copper mattes. With regard to the former, reference is made to U.S. Pat. Nos. 3,933,975, No. 3,933,976 and No. 4,034,059, among others. As regards the leaching of nickel and nickel-copper concentrates or mattes, reference is made to U.S. Pat. Nos. 3,293,037, No. 3,741,752 and No. 3,962,051.
The nickel leach solution obtained from the foregoing nickeliferous materials usually contains cobalt which is generally removed in order to provide a high purity nickel solution, for example, a solution having a nickel-to-cobalt ratio of over 1000:1. One method for removing the cobalt from solution as a cobaltic hydroxide precipitate is disclosed in U.S. Pat. No. 3,933,976.
The ratio of Ni/Co in the precipitate is normally about 2 to 5 (and may range as high as 10:1). After the cake has been washed (repulped) with water or acidified water (pH about 2.5), the Ni/Co ratio is improved and normally averages about 0.5 to 1.5. The precipitate is then further processed to reclaim the contained nickel values therein and to obtain a pure marketable cobalt product.
In order to refine further the cobaltic precipitate, it has to be dissolved or leached which is not easily accomplished. One method which has been proposed is that disclosed in U.S. Pat. No. 3,933,975. According to this patent, the cobalt black is leached with strong ammonia-ammonium sulfate solution at elevated temperatures of 180.degree. F. to 300.degree. F. (82.degree. C. to 149.degree. C.). While the method is commercially acceptable, a drawback is that the leach residue presents a considerable filtration problem. Moreover, the dissolution tends to be incomplete. A complete dissolution is a highly desirable goal because of the high market value for cobalt.
Substantially complete dissolution can be achieved by employing sulfuric acid in the presence of SO.sub.2 gas. However, the cobalt metal produced from the H.sub.2 SO.sub.4 --SO.sub.2 leach product usually contains intolerably high sulfur ranging from about 0.4% to 1% S, generally in the form of cobalt sulfide.
Sulfuric acid dissolution in the presence of metallic reductants (e.g. Co, Ni, Fe, Zn) instead of SO.sub.2 has been proposed but this process has not been very desirable because of the introduction of foreign ions (e.g. Fe, Zn) into the processing streams. The use of metallic nickel or cobalt as a reductant, while compatible with the process, adds to the production cost.
The invention overcomes the aforementioned disadvantage in that substantially complete dissolution can be attained; the dissolution residue can be easily filtered; the method of the invention is more economical than the system H.sub.2 SO.sub.4 --SO.sub.2 and also a low sulfur cobalt metal is obtainable.